They do get disabled in certain circumstances, but they should come back as you work. Saving, activating the window and some other things will 'reset' them so they won't work until new actions are taken. Then you should be able to undo back to the last reset.
Mark Steffke was correct. i had 2X protk.dat files and i edited the wrong one. i renamed to XXXX.dat.old and all is good.
I seem to remember that if you have toolkit apps registered, it disables the undo/redo. Im about 80% sure on that. Makes sense because toolkit apps could do any number of things to parameters or geometry, triggered from events, and pro/e couldn't track that if they wanted to.